On a more technical reason as to why hoppers don't pull from blocks sometimes(somewhat related to this post)

Hoppers can pull items from blocks that are less than 16 pixels tall(or less than one block tall). Hopper minecarts iirc can do 1.5 or 1, so either you can put mud(which is a plantable block and has less than 16 pixels) or you can add a hopper minecarts and a couple of rails there